The Structure of the Earth and Plate Tectonics

Structure of the Earth The Earth is made up of 3 main layers: –Core –Mantle –Crust Inner core Outer core Mantle Crust

The Crust This is where we live! The Earth’s crust is made of: Continental Crust - thick (10-70km) - buoyant (less dense than oceanic crust) - mostly old Oceanic Crust - thin (~7 km) - dense (sinks under continental crust) - young

What is Plate Tectonics?

If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle.

Plate Tectonics The Earth’s crust is divided into 12 major plates which are moved in various directions. This plate motion causes them to collide, pull apart, or scrape against each other. Each type of interaction causes a characteristic set of Earth structures or “tectonic” features. The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.

World Plates

What are tectonic plates made of? Plates are made of rigid lithosphere. The lithosphere is made up of the crust and the upper part of the mantle.

What lies beneath the tectonic plates? Below the lithosphere (which makes up the tectonic plates) is the asthenosphere.

Earth’s Layers How are the earth’s layers similar to an egg? Shell=crust Egg white=mantle Yolk=core

Plate Movement “Plates” of lithosphere are moved around by the underlying hot mantle convection cells

Convection currents Movement of the liquid-like Mantle move the relatively thin crust floating on it like ocean currents affect boats.

How do we know what the Earth is made of? Geophysical surveys: seismic, gravity, magnetics, electrical, geodesy –Acquisition: land, air, sea and satellite –Geological surveys: fieldwork, boreholes, mines

Practical Exercise 1 Supercontinents!

What happens at tectonic plate boundaries?

Divergent Convergent Transform Three types of plate boundary

New curst is created. Spreading ridges –As plates move apart new material is erupted to fill the gap Divergent Boundaries

Sonar Sonar pictures of the Mountain Ranges (baseball stitches)

Age of Oceanic Crust Courtesy of

Spreading Center

East African Rift Valley

Rift Valley Rift Valley and Oceanic ridges with NEW rock in the middle and OLD rock as you spread out. Rift Valleys continue to spread and sink until they get covered with water.

Magnetic reversal patterns are the same on both sides of the rifts. Magnetic Reversals

Iceland has a divergent plate boundary running through its middle Iceland: An example of continental rifting

Old crust destroyed by subduction There are three styles of convergent plate boundaries –Continent-continent collision –Continent-oceanic crust collision –Ocean-ocean collision Convergent Boundaries

Forms mountains, e.g. European Alps, Himalayas Continent-Continent Collision

Himalayas

Called SUBDUCTION Continent-Oceanic Crust Collision

Oceanic lithosphere subducts underneath the continental lithosphere Oceanic lithosphere heats and dehydrates as it subsides The melt rises forming volcanism E.g. The Andes Subduction

When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. The worlds deepest parts of the ocean are found along trenches. –E.g. The Mariana Trench is 11 km deep! Ocean-Ocean Plate Collision

Crust is neither created or destroyed. Where plates slide past each other Lots of EQ in CA. Transform Boundaries Above: View of the San Andreas transform fault

Practical Exercise 2 Where will the UK be in: 1,000 years? 1,000,000 years? 1,000,000,000 years?

…what’s the connection? Volcanoes and Plate Tectonics…

Volcanism is mostly focused at plate margins Pacific Ring of Fire

- Subduction - Rifting - Hotspots Volcanoes are formed by:

Pacific Ring of Fire Hotspot volcanoes

Hot mantle plumes breaching the surface in the middle of a tectonic plate What are Hotspot Volcanoes? Photo: Tom Pfeiffer / The Hawaiian island chain are examples of hotspot volcanoes.

The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. The volcanoes get younger from one end to the other.

…what’s the connection? Earthquakes and Plate Tectonics…

As with volcanoes, earthquakes are not randomly distributed over the globe At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur. Figure showing the distribution of earthquakes around the globe

Where do earthquakes form? Figure showing the tectonic setting of earthquakes

Plate Tectonics Summary The Earth is made up of 3 main layers (core, mantle, crust) On the surface of the Earth are tectonic plates that slowly move around the globe Plates are made of crust and upper mantle (lithosphere) There are 2 types of plate There are 3 types of plate boundaries Volcanoes and Earthquakes are closely linked to the margins of the tectonic plates